docs: def is_spontaneous(gene, custom_id=None):
      """Input a COBRApy Gene object and check if the ID matches a spontaneous ID regex.
      
      Args:
          gene (Gene): COBRApy Gene
          custom_id (str): Optional custom spontaneous ID if it does not match the regular expression ``[Ss](_|)0001``
  
      Returns:
          bool: If gene ID matches spontaneous ID
  
      """
  
      spont = re.compile("[Ss](_|)0001")
      if spont.match(gene.id):
          return True
      elif gene.id == custom_id:
          return True
      else:
          return False